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25.2 Photoelectric emission of electronsUttam Shrestha1HistoryEinstein won the Nobel Prize for Physics not for his work on relativity, but for explaining the photoelectric effect. He proposed that light is made up of packets of energy called photons. Photons have no mass, but they have momentum and they have an energy given by:Energy of a photon : E = hf

Uttam ShresthaPhoto electric effectThe photoelectric effect works like this. If you shine light of high enough energy on to a metal, electrons will be emitted from the metal. Light below a certain threshold frequency, no matter how intense, will not cause any electrons to be emitted. Light above the threshold frequency, even if it's not very intense, will always cause electrons to be emitted.

Uttam ShresthaThreshold frequencyThere is a certain amount of energy required to eject out an electron from metal surface. That amount of energy(work function),if it is at the least(minimum) amount which can bump out an electron, here the frequency of that energy is threshold frequency.The minimum frequency of the photon that is required to bump out an electron from the electron sea of the metal surface is called threshold frequency.

Uttam ShresthaPhotonPhoton is a particle representing a quantum(packet) of light or other electromagnetic radiation. Thephoton energyis theenergycarried by a singlephotonwith a certainelectromagnetic wavelengthandfrequency. It is represented by theelectron volt(eV) and thejoule

Uttam Shrestha5Energy of a Photon

ORWhere:h is Plancks constant = 6.63 10-34 Jsf is the frequency of the electromagnetic wave Is the wavelength of the electromagnetic wavec is the speed of light in vacuum = 3.00 108 ms-1Uttam ShresthaWork Function

In order to be ejected, the electron must absorb a certain amount of energy .We call this energy the work function of the material and it can be defined as the minimum energy necessary to remove an electron from the surface of the emitter material.Different metals have different work functions. This means it is easier to remove electrons from some metals than others.

Uttam ShresthaExplain why the maximum photoelectric energy is independent of intensity, whereas the photoelectric current is proportional to intensity

Uttam Shrestha8Relation of emission with frequency and intensityMetal FoilUttam ShresthaMetal FoilUttam ShresthaUttam ShresthaAs blue light strikes the metal foil, the foil emits electrons.Blue light has more frequency than red light as it has shorter wavelength than red light.

Uttam ShresthaUttam ShresthaNo matter how many photons strike the metal, if none of them has sufficient energy to eject an electron from a metal atom, you won't get a current.

Uttam ShresthaUttam ShresthaStill not working.What happens with brighter blue light?Uttam ShresthaUttam ShresthaMore blue light means more electrons emitted, but that doesnt work with red.If the energy the taken up by the electron is sufficient to allow it to be released from the metal atom, you will get a current.

Photoelectric EffectUttam ShresthaUttam ShresthaIncreasing the light intensity simply increases the emission of photoelectrons and the photocurrent. Uttam ShresthaThink about hitting a ball into outer space.If you don't hit it hard enough, it will just come back down. No matter how many times you hit it.

Uttam ShresthaTHANK YOUUttam Shrestha